1. Field of the Invention
The present invention relates to the field of high-speed gun or artillery launched projectiles. Particularly the present invention relates to air-breathing propulsion assisted projectiles, and more particularly to air propulsion assisted projectiles that accelerate after launch. Even more particularly the present invention relates to fuel release systems used in air breathing propulsion systems that accelerate after launch.
2. Description of the Prior Art
The ramjet and supersonic ramjet propulsion cycles for supersonic and hypersonic engines are well known within the art of aerospace propulsion. In ramjet propulsion high velocity air is compressed through a series of forebody and inlet shocks and through a subsonic diffuser all of which decelerate the air to a subsonic velocity near the fuel flame speed. Fuel is injected into a combustor and conventional subsonic combustion increases the temperature and pressure of the fuel-air mix. The high-pressure gas is then expanded through a nozzle increasing the velocity and momentum of the flow to produce thrust. Ramjet can be efficiently used to a velocity of approximately Mach 5. Above Mach 5 the temperatures and pressure associated with decelerating the flow to subsonic speeds for combustion are severe and begin eroding the engine cycle and the engine structure. It is at this point when supersonic combustion ramjet, called scramjet, is the preferred form of propulsion.
For Mach numbers above 5, a principal advantage of scramjet propulsion is that supersonic velocities within the combustion chamber are accompanied by lower static temperatures, pressures, and reduced total pressure losses. By reducing combustion product dissociation, reduced temperatures increase combustion efficiency, reduced pressures decrease loads on engine structure, and reduced total pressure losses increase the flow energy available for thrust production.
Research in supersonic air breathing propulsion systems for aircraft and missiles has been in progress since the 1940""s. As empirical knowledge grew on the subject in the 1950""s, researchers investigated propulsion for hypersonic aircraft and missiles, using scramjet engines. Research into scramjet propulsion continued during the 1970""s at the Nasa Langley Research Center and John Hopkins Applied Physics laboratory, and in the 1980""s and 1990""s work continued under the auspices of the National Aerospace Plane Program.
Starting in 1993 the Super High Altitude Research Project (SHARP) launched hypersonic air breathing vehicles for the purpose of data development on SCRAM propulsion. SCRAM propulsion has been discussed for several decades and is the cornerstone of many advanced vehicle concepts. The projectiles were launched using the SHARP hypervelocity launcher, which is a two-stage light gas gun. Because the flight duration was short, high specific impulse was required from the engine in order to produce a measurable deviation from pure ballistic flight. This lead to the selection of gaseous hydrogen as the fuel. One drawback of hydrogen is its low energy density. This, coupled with the low fuel volume available on the projectile, means that hydrogen must be stored at high pressure, e.g. 6000-10000 psi, and must not be prematurely released, i.e. before the projectile exits the launch tube. Using the SHARP light gas gun, velocities of up to MACH 9 have been recorded. The greatest advantage of the light gas gun launch is that high Mach number and high Reynolds numbers can be achieved simultaneously in invitiated air (clean). This guarantees that the flow field around a properly scaled model will match that of a full-scale hypersonic vehicle at operational speed and altitude. The primary disadvantage of the gun is that in gun launch there is a high axial acceleration load. In the SHARP test this can exceed 20,000 xe2x80x9cgxe2x80x9d. For this reason it is necessary that a robust mechanical design be implemented for launching the device.
Several devices have been created for use as projectiles for launch from a light gas gun. These devices take advantage of the characteristics inherent in scramjet and ramjet technology, and select hydrogen for its projectile fuel source. Hydrogen is selected as a gas source because, in order to achieve a measurable deviation from ballistic flight, an engine must produce high specific impulse, which is attainable using hydrogen.
U.S. Pat. No. 5,485,787 (1996, Bowcutt et al.) discloses a gas gun launched propulsion assisted scramjet projectile adapted to be fired from a gun preferably at velocities greater then MACH 5. The projectile includes a body with an internal combustion section, i.e. combustor, an external compression section, a nozzle section, and means for channeling fuel to the combustor to produce thrust greater than drag when the projectile travels at velocities greater then Mach 5. The projectile further includes a plurality of circumferentially spaced stabilization fins located at the nozzle end of the body. In addition the device includes a pusher for launching the device and protecting it from propulsive forces of the launch. One disadvantage of this device is that it is prone to fuel leakage and premature activation of the fuel system. A properly functioning fuel source is extremely important because of the low energy density of hydrogen gas. In addition, the projectile does not have provisions for repetitive cycling of the mechanism and testing before launch.
U.S. Pat. No. 5,513,571 (1996, Grantz et al.) discloses an air breathing propulsion assisted projectile designed to be rocket or gun launched and capable of accelerating to hypersonic velocities. This design includes a body having an encompassing cowl, an air compression section, an engine assembly located adjacent the air compression section, and a nozzle section located adjacent the engine assembly. The engine assembly includes apparatus for fuel storage and delivery to a combustion region. The rear end portion of the cowl is configured to direct the exiting combusted air and fuel mixture over the nozzle section of the body.
A scramjet system launched from a light gas gun for scramjet propulsion testing and experiments in a closed test chamber was documented in 1968 by H. H. King and O. P. Prachar in the Air Force Aero Propulsion Laboratory Technical Report AFAPL-TR-68-9. This study represents an early attempt to launch a scramjet-shaped projectile from a gun barrel, and the projectile was too small to contain a fuel source. The experiments were conducted only to assess the flight characteristics of scramjet models. Fuel sources were tested but only in conical shaped forms that did not constitute the principles of scramjet or ramjet technology.
All of these current projectile designs face the significant problem of utilizing their fuel source efficiently. The efficient use of hydrogen is significant because of its low energy density and the low fuel volume available on the projectile. Thus, it is critical that the fuel source is activated at the correct time and that all fuel is combusted. Inefficient fuel use leads to decreased projectile performance.
Therefore, what is needed is a scramjet projectile that incorporates a fuel release mechanism where the projectile design is able to withstand the high acceleration loads of a gun launch. What is further needed is a projectile powered by scramjet propulsion with a fuel activation source that is activated at a correct and consistent time after the projectile has left the gun muzzle. What is still further needed is a projectile that activates the fuel source without leaking or wasting uncombusted fuel.
It is an object of the present invention to provide an air breathing propulsion assisted projectile capable of travel at hypersonic velocities that will overcome some of the deficiencies and drawbacks of currently known air breathing propulsion assisted projectiles. It is another object of the present invention to provide a novel air breathing propulsion assisted projectile that is capable of acceleration by scramjet combustion operation at hypersonic velocities. It is yet another object of the present invention to provide a fuel injection mechanism that will activate only after the projectile has left the launcher muzzle and will not activate prematurely. It is a further objective of the present invention to provide a fuel activation mechanism that does not leak fuel from the projectile body.
The present invention achieves these and other objectives by providing a hypersonic projectile assembly having a main body, a cowl surrounding the main body, a nozzle formed by the rear end surfaces of the cowl and body, a forebody, and a nosecone. The main body contains a fuel cavity and a fuel activation system that is triggered by air stagnation pressure. The projectile assembly also includes a pusher that engages the aft end of the projectile. The pusher forms a seal between the projectile and the high pressure gas that propels it down the launch tube, and safely transfers the gun acceleration force to the projectile mechanical structure. To optimize specific impulse in scramjet propulsion, the above components must be designed with the following parameters considered: forebody and inlet contraction ratios, inlet efficiency, the fuel mixing efficiency, the combustor efficiency, and the nozzle efficiency.
The nosecone and forebody of the present invention contain an air intake port and channel, which is part of the fuel activation system, leading to a fuel injection activation mechanism. This feature distinguishes the present invention from all known projectiles of similar type. The channel created by the intake is referred to as the xe2x80x9cpitotxe2x80x9d tube.
Attached to the forebody is the main body of the projectile that houses the fuel cavity, the fuel activation mechanism, and a plurality of fuel distribution channels and fuel injection orifices that lead to the combustor region. Inside the body of the projectile, which is constructed of a metal such as aluminum, is the fuel cavity. The fuel activation mechanism retains the fuel within the fuel cavity. The fuel activation mechanism includes a fuel release piston and a fuel activation pin stop. The fuel activation pin stop is the rearward most portion of the fuel release piston. The fuel release piston, which is positioned within the fuel cavity, seals the fuel cavity to ensure that the fuel does not leak from the fuel cavity and releases fuel into the fuel distribution chamber when activated. To perform these functions, the fuel release piston includes at least two communicably attached sealing members, a forward fuel sealing member and a rear piston member. The forward fuel sealing member seals the fuel within the fuel cavity and prevents the fuel from entering into the fuel distribution chamber until required, i.e. until the projectile leaves the muzzle of the gun. The rear piston member, on the other hand, seals the capillary fueling channel once the fuel release piston is activated during launch. The rear piston member is attached to the channel sealing member, which may or may not be an integral part of the fuel release piston. The rear channel sealing member contains a capillary fixedly attached to the base for fueling the projectile. A taper in the fuel activation mechanism ensures that the piston seats forward during fuel charging. A third sealing member may also be employed for sealing the pitot tube to ensure that the air from the pitot tube and fuel do not mix before the fuel reaches the combustor region of the projectile.
In it""s most basic form, the fuel release piston must have a fuel sealing member with a forward surface that acts as the surface on which the air pressure supplied by the pitot tube is applied to activate the fuel system. This may be the pitot tube sealing member if employed or the forward fuel sealing member.
Pressure on the channel sealing member ensures that the forward fuel sealing member of the fuel release piston does not displace prematurely, thus compromising fuel release. The rear piston member is communicably attached to the channel sealing member, which is positioned within a channel that extends into the fuel cavity from the rearward end of the projectile body. The channel sealing member transmits pressure to the fuel release piston, which is communicably attached to the forward fuel sealing member, restricting any movement by the fuel release piston. The channel sealing member rests against the pusher or sabot, which is fitted to the rearward end of the projectile body.
The pusher is fitted to the body only during the in-bore phase of projectile launch. When the projectile and pusher exit the muzzle, drag separates the pusher from the projectile, freeing the channel sealing member, which in turn frees the fuel release piston, thereby allowing stagnation pressure to activate the fuel supply.
The effectiveness of the fuel activation system is very important due to limited fuel storage available on scramjet projectiles. The thrust required for projectile flight at speeds of approximately Mach 9 dictates that the fuel source be hydrogen or like gasses. Gasses such as hydrogen and the like have a low energy density. The low energy densities and minimal storage inherent in high velocity projectile flight dictate that fuel activation be achieved with little waste of fuel. The fuel activation system must initiate fuel delivery at a consistent and correct time. The correct time to initiate fuel activation is the moment the projectile exits the gun muzzle. Activation of the fuel source must also be achieved without leaking fuel from the fuel cavity. After activation the fuel is then routed through the fuel distribution plenum from the fuel cavity to the combustor region of the projectile in which the fuel is mixed with air and ignited.
In free flight, the fuel release piston is subject to primarily four forces. The pressure load (easily calculated from the piston differential cross sectional area and the fuel fill pressure), the o-ring static friction load and the inertial load (estimated from the aerodynamic drag and piston mass), all act forward. The projectile nose is in xe2x80x9ccleanxe2x80x9d air so that the pitot or stagnation pressure, which acts aft, is found from the Rayleigh formula or, alternatively, from the normal shock tables. Once the forces are known, it is a straightforward matter to calculate piston acceleration and the time required to initiate fuel flow past the forward fuel sealing member, through the injectors and into the combustor region. Computational fluid dynamics modeling is not required to predict performance. Though, in the present case, some modeling was performed to ensure that the small normal area presented by the pitot tube inlet would not affect inlet capture or significantly increase projectile drag. This mechanism is made possible by the non-linear scaling of pitot pressure with Mach number and would be less appropriate for more conventional projectile velocities. But at hypersonic velocities, a small opening, which presents minimal cross sectional area to the external air flow, can provide sufficient force to mechanically actuate fuel flow.
The use of pitot pressure at hypersonic velocities is not limited to activation of gaseous fuel flow. Pitot pressure may serve to drive a piston or a diaphragm for the purpose of injecting a liquid fuel. Pitot pressure may also be used to drive other mechanical processes such as the post-launch deployment of control surfaces, e.g. fins and wings. The pitot tube, in conjunction with a transducer and telemetry unit, can concurrently be used to measure projectile velocity.
The channel sealing member in conjunction with the fuel release piston and the pitot tube solve the problems of current projectile designs with respect to eliminating premature fuel release and fuel leakage. The fuel release piston slides into the fuel cavity sealing the fueling capillary channel and releasing the fuel into the combustor when activated by the air flow pressure from the pitot tube. Pressure on the channel sealing member ensures that the fuel release piston is not displaced prematurely. The materials selected allow a high strength weld for making a leak-proof attachment of the fueling capillary to the channel sealing member.
In the embodiment described above the use of only two sealing surfaces allows for some mixing of air and fuel within the fuel distribution lines at the end of the pitot tube. To stop this from occurring, the second embodiment includes a pitot tube sealing member communicably attached to the fuel release piston. The pitot tube sealing member is positioned in between the aforementioned forward fuel sealing member and the air intake pitot tube. In this embodiment the pitot pressure is placed on the face of the pitot tube sealing member and the force is then communicated to the fuel sealing member, which consistent with the first embodiment seals the fuel within the fuel cavity. The pitot tube sealing member seals the pitot tube in front of the fuel distribution lines, and as a result ensures that the air in the pitot tube does not mix with the fuel in the fuel distribution chamber. This is the preferred solution for hydrogen fuel. Without the pitot tube sealing member, the hydrogen fuel, which is at much higher pressure than the pitot stagnation pressure, will spill out of the pitot tube. This produces pressure thrust aft, in the wrong direction. Further, the hydrogen spilling from the nose auto-ignites, which disrupts the flow to the scram inlet. In certain situations, however, such as where solid fuels are used, pre-mixing of fuel and air to partially burn a solid fuel xe2x80x9cgas generatorxe2x80x9d to inject fuel-rich partially combusted gases into the scramjet combustor may be the preferred method.
A third embodiment of the present invention includes a removable nosecone for repetitive cycling tests of the mechanism before launch. To simulate pitot pressure, the nosecone is unscrewed and replaced with a fitting that allows the charged projectile to be connected to a gas cylinder. This allows non-destructive testing of the fuel release mechanism under expected flight stagnation pressure conditions. In practice, fuel flow could be initiated in this manner at pressures within a few percent of design predictions.
Additional advantages and embodiments of the present invention will be set forth in part in the detailed description that follows and in part will be apparent from the description or may be learned by practice of the invention. It is understood that the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.